Method and apparatus for high density nanostructures

ABSTRACT

A method and apparatus for high density nanostructures is provided. The method and apparatus include Nano-compact optical disks, such as nano-compact disks (Nano-CDS). In one embodiment a 400 Gbit/in 2  topographical bit density nano-CD with nearly three orders of magnitude higher than commercial CDS has been fabricated using nanoimprint lithography. The reading and wearing of such Nano-CDS have been studied using scanning proximal probe methods. Using a tapping mode, a Nano-CD was read 1000 times without any detectable degradation of the disk or the silicon probe tip. In accelerated wear tests with a contact mode, the damage threshold was found to be 19 μN. This indicates that in a tapping mode, both the Nano-CD and silicon probe tip should have a lifetime that is at least four orders of magnitude longer than that at the damage threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/107,006 filed Jun. 30, 1998 (now U.S. Pat. No. 6,309,580issued Oct. 30, 2001) which, in turn, is a continuation-in-part of U.S.patent application Ser. No. 08/558,809 filed Nov. 15, 1995 (now U.S.Pat. No. 5,772,905 issued Jun. 30, 1998). This application also claimsthe benefit of U.S. Provisional Application Ser. No. 60/106,475 filed onOct. 30, 1998, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates generally to fabrication ofnanostructures and in particular to high density nanostructurefabrication using nanoimprint lithography.

BACKGROUND OF THE INVENTION

Nanostructures are used for a variety of application areas, including,among other things, optical and magnetic data storage. One form of datastorage is a lowcost information storage media known as Read Only Memory(ROM). One way to make ROM disks is by injection molding. Such disks mayhave a data storage density of ˜0.68 Gbit/in², and are read using afocused laser beam. To meet the future demand for ROM disks withincreasing information storage densities, methods must be developed forlow-cost manufacturing of such disks with replicated data patterns, andfor inexpensive read-back techniques suitable for retrievinghigh-density information.

One attempt is to develop ROM disks with ultrahigh-density topographicalbits and to use proximal-probe based read-back. ROM disks of topographicbits with 45 Gbit/in² storage density have recently been reported by agroup from IBM (B. D. Terris, H. J. Mamin, and D. Rugar, 1996 EIPBN,Atlanta, Ga., 1996; B. D. Terris, H. J. Mamin, M. E. Best, J. A. Logan,D. Rugar, and S. A. Righton, Apply. Phys. Lett., 69, 4262 (1996)). Thisgroup reports that features as small as 50 nm were produced by electronbeam lithography and replicated on a glass substrate using aphotopolymerization (2P) process. However, a smaller the feature size isneeded to increase the storage density of the medium.

What is needed in the art is an improved method and apparatus for highdensity nanostructures. There is also a need for smaller feature sizestorage to enhance storage density.

SUMMARY OF THE INVENTION

The present disclosure teaches methods and apparatus which address theneeds in the art mentioned above and addresses several other needs notmentioned expressly herein, but appreciated by those skilled in the art.

Method and apparatus for producing nanostructures is provided. Thenanostructures are useful in the production of high density andultra-high density storage media. The method and apparatus aredemonstrated in the application to nano-compact disks, however, themethod and apparatus are suitable for other applications, and thenano-compact disk application is not intended in an exclusive orlimiting sense.

In particular nano-compact disks with 400 Gbit/in² storage densitycontaining 10 nm minimum feature sizes have been fabricated usingnanoimprint lithography. Furthermore, method and apparatus relating tothe reading and wearing of Nano-CDS using scanning proximal probetechniques are described. This storage density is nearly three orders ofmagnitude higher than commercial CDS (0.68 Gbit/in²). Other embodimentsare possible with different feature sizes and different storagedensities using the method and apparatus provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of one nanoimprint lithography process forproduction of nanostructures according to one embodiment of the presentsystem.

FIG. 2 is a SEM micrograph of a 50 nm track with Nano-CD daughter moldfabricated using nanoimprint lithography, according to one embodiment ofthe present system.

FIG. 3 is a SEM micrograph of a 40 nm track width Nano-CD fabricatedwith nanoimprint lithography and liftoff, according to one embodiment ofthe present system.

FIG. 4 is a SEM micrograph of a Nano-CD consisting of 10 nm metal dotswith a 40 nm period fabricated using nanoimprint lithography andliftoff, according to one embodiment of the present system.

FIG. 5 is an initial tapping mode AFM image (a) and 1000th image (b) ofa Nano-CD consisting of 50 nm period gold dots fabricated usingnanoimprint lithography and liftoff, according to one embodiment of thepresent system.

FIG. 6 shows cross sections of contact mode AFM images showing wear ofchrome grating after various applied forces using a silicon scanningprobe tip, according to one embodiment of the present system. The imagesare for (a) initial, (b) 11 μN, (c) 15 μN, and (d) 19 μN applied force.Only at the 19 μN force the tip removes the Cr grating.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views.

One embodiment of the present system uses a nanostructure fabricationprocess incorporating nanoimprint lithography (NIL) to create highdensity storage media, such as optical disks, for example compact disks.Other high density or ultra high density storage formats, such asmagnetic, are possible without departing from the scope of the presentsystem.

NIL is a high-throughput and low-cost nonconventional lithographytechnology with sub-10 nm resolution. One embodiment of the technologyis provided in FIG. 1, and is discussed in U.S. Pat. No. 5,772,905 by S.Y. Chou, and the articles by S. Y. Chou, P. R. Krauss, and P. J.Renstrom, in Applied Physics Letters, 67, 3114 (1995); and Science, 272,85 (1996), all of which are incorporated herein by reference in theirentirety. Other embodiments and applications are described in copendingU.S. patent application Ser. No. 09/107,006, entitled Release Surfaces,Particularly for Use in Nanoimprint Lithography, and in U.S. Pat. No.5,820,769, Ser. No. 08,448,807 entitled Method for Making MagneticStorage Having Discrete Elements With Quantized Magnetic Moments, andcopending U.S. patent application Ser. No. 08/762,781, entitled QuantumMagnetic Storage, all of which are incorporated by reference in theirentirety. Applicant also incorporates by reference in its entirety thearticle entitled Nano-compact disks with 400 Gbit/in² storage densityfabricated using nanoimprint lithography and read with proximal probe byP. R. Krauss and S. Y. Chou in Applied Physics Letters. 71 (21), Nov.24, 1997.

In one embodiment, NIL patterns a resist through deformation of resistphysical shape by embossing rather than through modification of resistchemical structure by radiation or by self-assembly. The nanoscaletopographical bits on a Nano-CD can be made with a variety of materialssuch as polymers, amorphous materials, crystalline semiconductors, ormetals. Here, we focus our discussion on one embodiment of Nano-CDSconsisting of metal bits. Other embodiments and applications arepossible, and the description herein is not intended in a limiting orexclusive sense.

In this embodiment, the first step of the Nano-CD fabrication processuses a SiO₂ mold on a silicon substrate with a CD-like data patternfabricated using high-resolution electron beam lithography and reactiveion etching. The SiO₂ was selected because it has a low atomic number toreduce the backscattering and proximity effects during the electron beamlithography, thereby extending the lithography resolution down tofeatures as small as 10 nm with a 40 nm period, in one embodiment. Otherembodiments having different feature sizes are possible withoutdeparting from the present system. Although high-resolution electronbeam lithography is a relatively expensive and low-throughput process,the master mold may be used to replicate many Nano-CDS using inexpensiveand high-throughout NIL. Furthermore, the master mold may be used tofabricate daughter molds, thereby increasing the total number of disksthat can be fabricated per master mold, and lowering the cost per disk.The daughter molds may be composed of the same material as the mastermold, or other materials (such as high atomic number materials) that areoptimized for better durability performance. A daughter mold with 13 nmminimum feature size and 40 nm pitch fabricated using NIL, is shown inFIG. 2. Other feature sizes with different minimum feature sizes arepossible without departing from the present system.

The second step in the Nano-CD fabrication process, according to thisembodiment, was to imprint the mold into a polymer resist film on a disksubstrate using NIL. The 75-nm-tall SiO₂ master Nano-CD mold wasimprinted into a 90-nm-thick polymethyl-methacrylate (PMMA) film on asilicon disk. During the imprint step, both the mold and resist coateddisk were heated to 175° C., however, other temperatures are possiblewithout departing from the present system. The mold and wafer werecompressed together with a pressure of 4.4 MPa for 10 minutes at thistemperature, followed by being cooled down to room temperature. The moldwas then separated from the disk resulting in duplication of the Nano-CDdata pattern in the PMMA film. A mold release agent, as described inU.S. patent Ser. No. 09/107,006, entitled Release Surfaces, Particularlyfor Use in Nanoimprint Lithography, which was incorporated by referencein its entirety, may be used to improve the resolution of the imprintingand improve the minimal feature size. Furthermore, it has beendemonstrated that using a single molecular layer of release agent oragents may provide a minimal feature size of 10 nanometers or less.

At this point, it is possible to directly use the disk with thepatterned PMMA for data read-back, such as done with acrylate-based 2Pprocesses. One advantage of NL over the 2P process is that it canproduce smaller feature sizes. Another advantage is that the substratechoice in NIL is not limited to UV transparent materials such as glass,but can be silicon, aluminum, or other opaque substrates.

The third step of the Nano-CD fabrication process, according to thisembodiment, was to transfer the imprinted pattern into metal bits, whichhave much better durability than polymers during read-back. Ananisotropic 2 RIE pattern transfer step was used to transfer theimprinted pattern through the entire PMMA thickness. The resulting PMMAtemplate was used to transfer the Nano-CD pattern into metal using aliftoff process where Ti/Au (5 nm/10 nm thick) were deposited on theentire disk and lifted off. FIG. 3 shows a section of a Nano-CD with a40 nm track width and 13 nm minimum feature size, fabricated using themold shown in FIG. 2. Other minimal feature sizes are possible withoutdeparting from the present system. This track width corresponds to astorage density of 400 Gbit/in². FIG. 4 shows another 400 Gbit/in²Nano-CD with 10 nm minimum feature size and 40 nm pitch. Gold was chosendue to it high contrast on the silicon substrate in the scanningelectron microscopy (SEM). Other materials may also be used which offerbetter wear properties than gold, as discussed later.

In one embodiment, rather than deposit material on substrate the PMMAcan be used as the etch mask to directly etch the substrate.

It is noted that the fabrication process described herein is notintended in an exclusive or limiting sense. Other materials may be usedand temperatures and processes may be employed which are within thescope of the present system.

A high-resolution and nondestructive technique is needed to read datastored in the nanoscale topographical bits of a Nano-CD. The bits aretoo small to be read by current laser beams as used in CDS. In oneembodiment, information stored on Nano-CDS was read back using an atomicforce microscope (AFM) with commercial silicon scanning probes. Bothtapping mode and contact mode AFM were demonstrated. FIG. 5(a) shows atapping mode AFM image and a cross-section profile of a Nano-CDconsisting of a uniform array of gold dots with a 50 nm period. Tappingmode AFM images show the gold dots are wider than the 10 nm measured bySEM. The discrepancy is attributed to the scanning probe's tip size. Thecross-section profile indicates that the probe tip can resolveindividual nanoscale dots and the flat silicon substrate between the 50nm period dots. However, for 40 nm period dot arrays with the samediameter, the probe tip could not always reach the substrate, making thedot height measured by AFM smaller than that for 50 nm period dots. Thisproblem can be avoided by using a sharper probe.

The wear of Nano-CDS and the scanning probe during read-back process wasinvestigated. Tapping mode AFM (a force range of 0.1-1.0 nano-Newtons)was used to scan the same location of the Nano-CD 1000 times as shown inFIG. 5(b). We did not observe any discernible change in the AFM image.This indicates that neither the silicon proximal probe nor the Nano-CDexhibited significant wear during the tapping mode AFM imaging.

To accelerate the wear test of the tips and the disks, contact mold AFMand large tip forces were used. Moreover, the gold dots were replaced bya 15-nm-thick chrome grating of a 3 μm spacing and linewidth fabricatedusing photolithography and liftoff. Chrome has a Mohs hardness of 9,making it more resistant to wear than gold, which has a hardness of 2.5.The magnitude of the applied forces depends upon the spring constant ofthe proximal probe cantilever. The AFM tips used were 125-μm-longcommercial silicon cantilevers which had spring constants ranging from20 to 100 N/m. Since the spring constant of the cantilevers was notaccurately known, the approximate forces were calculated using a springconstant of 60 N/m.

FIG. 6 shows 10-μm-wide cross-section profiles from contact mode AFMimages of the chrome grating after various forces were applied to thecenter 5-μm-wide section. The AFM tip force can be increased to 15 μNwithout creating immediate noticeable change in the AFM image. However,at 19 μN force, the silicon tip will remove the chrome line duringscanning. This indicates that in tapping mode, where the AFM tip forcecan be over four orders of magnitude smaller than the damage threshold,both the Nano-CD and silicon probe tip should have a lifetime that is atleast four orders of magnitude longer than that at the damage threshold(although the exact relation between the wear and the force is unknown).High data retrieval rates may be obtained by using arrays of scanningprobe tips operating in parallel.

In one embodiment, another method of reading the data is to use a nearfield probe. A near field probe is a special type of optical tip withsub 100 nanometer resolution. In one embodiment, the data can also beread by using a capacitance probe. In such an embodiment, differentspacing gives different capacitances. Other embodiments are possiblewithout departing from the present system.

What is claimed is:
 1. A method for making high density nanostructures,comprising: fabricating a mold on a substrate, the mold having acircular data pattern; imprinting the mold into a polymer resist film byheating and compressing the mold and polymer resist film; cooling themold and polymer resist film; and removing the mold from the polymerresist film to provide a patterned surface.
 2. The method of claim 1,further comprising using one molecular layer of release agent andwherein the mold has a feature size of approximately 10 nanometers. 3.The method of claim 1, further comprising forming a nano-compact diskhaving the patterned surface.
 4. The method of claim 1, furthercomprising forming a nano-compact disk having the patterned surface,wherein the nano-compact disk has a storage density of approximately 400gigabits per square inch.
 5. The method of claim 1, further comprisingforming a storage media disk having the patterned surface.
 6. The methodof claim 1, further comprising forming a storage media disk having thepatterned surface, wherein the storage media disk has a storage densityof approximately 400 gigabits per square inch.
 7. The method of claim 1,further comprising forming a magnetic media disk having the patternedsurface, wherein the storage media disk has a storage density ofapproximately 400 gigabits per square inch.
 8. The method of claim 1,further comprising: etching residual resist in recessed areas; anddepositing a material according to the pattern which is durable duringread-back.
 9. The method of claim 8, further comprising using thepolymer resist pattern as the etch mask to the substrate.
 10. The methodof claim 8, wherein the material deposited according to the pattern is ametal.
 11. The method of claim 1, further comprising forming a magneticmedia disk having the patterned surface.
 12. The method of claim 11,further comprising: using the imprinted polymer resist film as an etchmask to the substrate; and depositing a magnetic material according tothe pattern.
 13. The method of claim 11, further comprising: etchingresidual resist in recessed areas of the imprinted resist film; usingthe remaining polymer resist pattern as an etch mask to the substrate;and depositing a magnetic material according to the pattern.
 14. Amethod, comprising: fabricating a mold on a substrate, the mold having acircular data pattern for nanoimprinting; creating one or more daughtermolds using the mold; and using the one or more daughter molds to createa patterned substrate.
 15. The method of claim 14, wherein the creatingthe one or more daughter molds using the mold further comprises:imprinting the mold into a polymer resist film by heating andcompressing the mold and polymer resist film; cooling the mold andpolymer resist film; and removing the mold from the polymer resist filmto provide a daughter mold template in the resist film.
 16. A methodcomprising: fabricating a mold on a substrate, the mold having acircular data pattern for nanoimprinting; creating one or more daughtermolds using the mold; and using one daughter mold of the one or moredaughter molds to create a patterned substrate by: imprinting the onedaughter mold of the one or more daughter molds into a polymer resistfilm by heating and compressing the daughter mold and polymer resistfilm; cooling the daughter mold and polymer resist film; and removingthe daughter mold from the polymer resist film to provide the patternedsurface.
 17. The method of claim 16, further comprising: etchingresidual resist in recessed areas; and depositing a material accordingto the pattern which is durable during read-back.
 18. The method ofclaim 16, further comprising using one molecular layer of release agentand wherein the mold has a feature size of approximately 10 nanometers.19. The method of claim 16, further comprising: etching residual resistin recessed areas; using the remaining polymer resist pattern as theetch mask to the substrate; and depositing a material according to thepattern which is durable during read-back.
 20. The method of claim 19,wherein the material deposited according to the pattern is a metal.